1. Field of the Invention
This invention relates generally to separation of gaseous mixtures by selective adsorption and is more particularly concerned with an adiabatic pressure swing adsorption system designed and operated for separate recovery from a multicomponent gas mixture of a primary key component and a secondary key component, each substantially freed of the other key component and of the dilute components present in minor quantity in the original gas mixture subjected to treatment. To effect such separate recovery of desired components of the feed gas mixture the system of the invention uses separate beds of adsorbent in concurrent series gas flow therebetween during the adsorption stage, yet designed for independent operation during the regeneration or desorption stages.
2. Prior Art
Pressure swing cyclic adsorption systems designed for fractionation of gaseous mixtures by selective adsorption are well-known in the art. In these systems one or more desired components of the feed gas mixture are separately recovered at a yield and purity depending upon the modes of the designed operation and their efficiency.
Illustrative of typical systems indicated to be especially useful in the recovery of hydrogen from gaseous mixtures with CH.sub.4 and/or CO.sub.2 are those described in U.S. Pat. Nos. 3,138,439; 3,142,547; 3,788,037. Other patents describe in general systems for separation of essentially binary gas mixtures or of multicomponent gas mixtures. Illustrative of these are the systems for separation of essentially binary gas mixtures or of multicomponent gas mixtures, asserted to be applicable in recovery of hydrogen from such mixtures. Illustrative of these are the systems described, for example, in U.S. Pat. Nos. 3,221,476; 3,430,418; 3,720,042. Also among the systems described in the prior patent art are those employing separate adsorbent beds operated in series flow and designed for, or stated to be applicable in, separate recovery of hydrogen and one or more other components present in a multicomponent feed gas mixture. Typical among such systems are those described in U.S. Pat. Nos. 3,102,013; 3,149,934; 3,176,444; 3,237,379; 3,944,400; and 4,000,990.
According to the present invention, a primary key component and a secondary key component are recovered in separate streams at high purity and in good yield from a multicomponent feed gas mixture, each stream being substantially free of minor dilute contaminants originally present in the feed gas mixture. Such separation and recovery of the desired components are accomplished by the hereinafter described sequence and mode of operation in a system comprising a plurality of trains of adsorbent columns continuously operated in timed cyclic sequence.
In U.S. Pat. No. 4,077,779, there are described adsorption systems designed primarily for separation of binary gas mixtures which may contain trace amounts of other impurities. While the systems therein described can be successfully operated in the separation of such binary gas mixtures, these systems cannot be efficiently utilized in the individual recovery of two key components from a multicomponent gas mixture containing in addition to these major key components a minor quantity (more than trace amounts) of one or more contaminating dilute components. The presence of such dilute components may adversely affect the efficiency of gas separation of pressure swing adsorption techniques designed for handling essentially binary gas mixtures. However, the dilute components are often present in such a small quantity that there is little incentive for their separate recovery in highly enriched form. Thus for many multicomponent gas mixtures confronted in industry for separation therefrom of desired primary key and secondary key components at high purity and yield, an impure tertiary stream of the dilute components or their presence in the recovered secondary product is often acceptable.
Multicomponent gas mixtures containing a bulk primary component, a bulk secondary component and one or more dilute components, generally encountered in industrial separation can be classified into two different groups:
(1) Such mixtures in which the minor dilute components are less strongly adsorbed than the secondary key component.
(2) Such mixtures in which the minor dilute components are more strongly adsorbed than the secondary key component.
An example of a mixture of the first type is the gaseous effluent from a shift converter in a hydrocarbon reforming plant. A typical composition of such effluent may be 76% H.sub.2, 20% CO.sub.2, 3.5% CH.sub.4, and 0.5% CO (each by volume). From such mixture CO.sub.2 is to be removed as the secondary key component and hydrogen recovered as primary component in substantially pure state. The dilute impurities such as CO and CH.sub.4, and N.sub.2 if present, are less strongly adsorbed than CO.sub.2 on most commercial sorbents such as activated carbons and certain molecular sieves. In separation of this kind of gas mixture, a preferred plan may be to obtain a stream of high purity H.sub.2 as the primary product, a pure stream of CO.sub.2 as the secondary product, and a tertiary stream containing the CO, N.sub.2 and CH.sub.4 impurities along with some H.sub.2 which can be burnt as fuel.
An example of a mixture of the second type is the effluent gas from a hydrodesulfurization plant after the removal of the sulfur compounds, wherein it is desired to remove CH.sub.4 to recover high purity recycle hydrogen. A typical desulfurized gas may contain for example: 65% H.sub.2, 20% CH.sub.4, and 5% each of C.sub.2, C.sub.3 and C.sub.4 -C.sub.6 hydrocarbon components (each by volume). In this instance also hydrogen constitutes the primary component. The C.sub.2 + hydrocarbons are more strongly sorbed than the secondary component (CH.sub.4). The dilute impurities containing C.sub.2 -C.sub.6 compounds in this case, may be tolerated in the secondary product stream.
The present invention is particularly concerned with multicomponent gas mixtures of the first kind hereinabove described. In such gas mixtures high recovery of the primary key component (hydrogen) is critical because any unrecovered hydrogen lost with the secondary key component product, mainly CO.sub.2, cannot be efficiently burnt.
Systems for handling gas mixtures of the second type above-described is the subject of the aforesaid companion application Ser. No. 935,424. In treating gas mixtures of the second type, high recovery of the primary component is not as critical economically since the hydrogen, which is not recovered in the primary product, becomes a part of the secondary product mainly containing CH.sub.4 and C.sub.2 + components which can be used as fuel gas of high value.
Among the objectives obtained by practice of the present invention is the economical individual recovery of both the primary and secondary key components at high purity and improved product yields as compared to the identified prior art systems.